Every aspect of Perl's operation has a related test, and these
test files live in the t/ directory. Tests
for individual library and XS modules are slowly being relocated
to lib/ and ext/
respectively, but at time of writing, there are over 23,000
separate tests in over 400 test files.

On a related note, functions for debugging Perl itself are to be
found in deb.c and
dump.c.
The distinction is that functions in deb.c
are typically accessible from the -D flag on
the Perl command line, whereas things in
dump.c may need to be used from a
source-level debugger.